1. Field of The Invention
The present invention relates generally to an air flow rate measuring system which uses a hot-wire type air flow meter for measuring flow rate of intake air introduced into an internal combustion engine or the like. More specifically, the invention relates to an air flow rate measuring system which can correct its response time lag when the engine operates in a transient state.
2. Description of The Background Art
As is well known, a so-called hot-wire type air flow meter has been widely used for controlling internal combustion engines. Such a hot-wire type air flow meter has a heating element made of platinum or nickel foil, and is designed to detect air flow rate by measuring heat dissipation caused by exposing the heating element to air flow within an intake pile. One hot-wire type air flow meter is disclosed in, for example, Japanese Patent First (unexamined) Publication (Tokkai Sho.) No. 48-103924.
In the aforementioned air flow meter, there is a disadvantage in that it is insufficiently responsive to variations of actual air flow rate, which causes response time lag when the engine operates in a transient state (when accelerated or decelerated).
In order to overcome the aforementioned problem, an improved air flow meter has been proposed in Japanese Patent Application (Tokugan Sho.) No. 63-310440. In this air flow meter, the response time lag caused in its output is presumed as a first-order lag of its approximate value, and the output value of the air flow meter is corrected by using variations of output value per a predetermined period of time.
Specifically, when abrupt variation of real air flow rate (instantaneous flow rate) Q occurs, the first-order lag of an output value Q.sub.shw of the air flow meter may be expressed by the following equation. EQU Q.sub.shw =Q{1-exp(-t/T)} (1)
in which t is an elapsed time from the abrupt variation, and T is a time constant of response.
Assuming that the output value Q.sub.shw of the air flow meter is sampled at every predetermined period of time, e.g. every 10 ms, as shown in FIG. 1, the aforementioned equation (1) can be approximated as follows, which shows a weighted mean of the output values Q.sub.shw of the equation (1). EQU Q.sub.shw =Q.times.K+oldQ.sub.shw .times.(1-K) (2)
in which K is a weighted mean factor (a value less than or equal to 1), and the word "old" of the old Q.sub.shw means the preceding value (10 ms before). "Old" will hereinlater be used in this same manner.
From the equation (2), the real air flow rate Q may be expressed by the following equation. EQU Q=oldQ.sub.shw +(Q.sub.shw -oldQ.sub.shw).times.(1/K) (3)
Now, assuming that an air flow rate measured by an air flow rate measuring system is Qs and that (1/K) is a correction coefficient A.sub.fmtc, an air flow rate in which any response time lags are corrected can be obtained by using an output value (old Q.sub.shw) of the air flow meter and a variation .DELTA.Q.sub.shw (=Q.sub.shw -old Q.sub.shw) per a predetermined period of time, in accordance with the following equation. EQU Qs=oldQ.sub.shw +.DELTA.Q.sub.shw .times.A.sub.fmtc ( 4)
By using this equation (4), it was found that an air flow meter has a superior responsiveness when the air flow meter has a thin platinum wire exposed to air flow.
However, platinum wires do not only have poor durability, but they are also very expensive. For that reason, in recent years, air flow meters often have a sensor portion which comprises either a heating element wound onto a ceramic bobbin as a coil or a film-shaped heating element mounted on a ceramic member.
When a hot-wire type air flow meter having a sensor portion of this type is applied to the aforementioned proposed system to measure air flow rate, it was found, by experiment, that there is a great divergence between the measured air flow rate and the actual air flow rate. FIGS. 2 and 3 show response waveforms, which are expressed in the drawings as corrected outputs, of the aforementioned proposed system when accelerated and decelerated, respectively. As shown in these drawings, divergences of the measured air flow rates from the real air flow rates become great immediately after leading edges (rising transition points) and trailing edges (falling transition points). In addition, when pulsation of intake air occurs in a full throttle state, variations of output value become greater than the pulsation of intake air.
For that reason, when the amount of fuel injected into an engine is derived in accordance with the corrected outputs as shown in the drawings, an air/fuel ratio of an air/fuel mixture introduced into the engine may deviate from the stoichiometric value so that drivability and exhaust gas purifying performance of the engine become bad.